The present invention relates to a device for filling in a trench dug in the sea bed in order to cover up a pipe laid down in the said trench.
In the sense of the present invention, pipe is understood to be a rigid or flexible tubular conduit, a conduit including a bundle of conduits of the umbilical type, an electric power cable or a cable for transmitting signals, or a collection of such conduits or cables arranged side by side.
It is often necessary to afford protection to underwater pipes by burying them below the surface of the sea bed, especially in order to protect them from objects likely to damage them such as the anchors of ships, trawling nets, etc., and to keep the pipe still while placing it out of reach of the action of currents and the swell.
The use in this context of machines known as burying machines is known, these making it possible to dig a trench in the sea bed by mechanically cutting into the sea bed and/or by deaggregating the latter using jets of pressurized water, the burying machines being towed from the surface or being self-propelled and controlled remotely from a vessel on the surface. The burying operation may be carried out at the same time as the operation of laying the pipe on the sea bed, as described for example in the prior patents of the filing company FR-A-2,455,235 and FR-A-2,475,681, or alternatively be carried out subsequently, after the pipe has been laid on the sea bed beforehand.
To the end of improving the protection of the pipe, the latter is commonly laid down a certain distance below the surface of the sea bed.
It is hence necessary to dig trenches in the sea bed, the depth of which trenches in practice is of the order of 1 meter, but may sometimes reach or exceed 2 meters depending on the diameter of the pipe to be buried and the desired depth of burying.
Depending on the characteristics of the ground forming the sea bed, hard or soft, and on the technique used to make the trench, the latter may exhibit a more or less narrow and deep substantially rectangular cross-section or quite an open V-shaped cross-section or alternatively any other intermediate shape.
The trench may in some cases be filled in spontaneously to a more or less partial extent, but insofar as the thickness of the material thus deposited, even though it may be relatively significant in places, varies unpredictably and in a way which cannot easily be monitored, it is necessary to ensure the total filling-in of the trench in a reliable fashion, particularly in the case of ground which is not very highly consolidated.
The complete covering of a pipe up to the level of the surface of the sea bed makes it possible, particularly in the case of pipes for conveying fluids such as hydrocarbons at high temperature and under high pressure, to limit the development of loops which might be formed by buckling owing to the increase in length of the pipe under the effect of temperature and/or pressure.
The burying of the pipe further makes it possible to improve its thermal insulation.
Various devices making it possible to fill in to a greater or lesser extent a trench dug in the sea bed are already known.
Thus, equipment is known which includes blades which scrape the sea bed over a certain width, like the civil engineering works machines of the "scraper" type. Such equipment makes it possible to clear and to convey, in order to deposit them in the trench, quantities of materials coming from the sea bed, particularly some of the spoil deposited at the sides on either side of the trench during the prior digging operation, as well as constituent parts of the ground extracted mechanically from the superficial part of the sea bed on either side of the trench.
Such equipment is bulky and heavy, expensive to produce and to use and requires high traction forces to move it along the trench to be filled in.
It has moreover been proposed (JP-A-56 59932) to fill in a trench dug for laying a conduit, by spraying pressurized water using two nozzles arranged on either side of the trench at the back of the burying machine, and pointing backwards so as to return towards the trench constituent parts of ground removed from the banks which have been formed on each side of the trench by the spoil deposited at the side when the said trench was being dug. The effectiveness of such a device is very slight and a very small, uneven and random proportion of the back-fill materials reaches the bottom of the trench in order to cover the conduit, which does not allow the said trench to be filled in satisfactorily, it even being possible, in some circumstances, for the device to widen the trench instead of filling it in.